1. Field of the Invention
The present invention relates to a probe microscope and a measuring method using the probe microscope, more particularly, to a probe microscope capable of measuring an electronic state density of a surface of a high-resistance material, and a measuring method using the probe microscope.
2. Description of the Related Art
In recent years, a significant increase in performance of an electronic device represented by a silicon device and a reduction in size of the structure thereof have been realized, and an electronic device has been developed to achieve a structure size in an order of nanoscale.
Not only an inorganic material but also an organic material and a polymer material are incorporated into the device as constituent materials.
When the device becomes a nanoscale size, it is necessary to use a method of locally evaluating electronic characteristics of a portion which exerts a predetermined function in the order of nanoscale. A method using contact electrification with metal has been reported as a method of measuring an electronic state density of a high-resistance material such as a polymer material in “Molecular charge states and contact charge exchange in polymers” Thomas J. Fabish and Charles B. Duke, et al., J. Appl. Phys. 48 (1977), 4256 and “Polymers as electronic materials” by C. B. Duke, J. Vac. Sci. Tecnol., 3(1985), 732.
This method is a method of bringing several kinds of metals such as Ni, Pb, Au, In, Sn, and Pt into contact with a polymer film to measure a change of a surface potential and estimating the electronic state density of the polymer material based on a result obtained by the measurement.
Up to now, an evaluation method using a combination of an X-ray photoelectron spectroscopy (XPS) or an ultraviolet photoelectron spectroscopy (UPS) and an inverse photoelectron spectroscopy (IPES) has been known as a method of measuring an electronic state of the surface of a sample.
For example, when an electronic structure of mainly a (vapor-deposited) thin film made of an organic compound, a metal complex, or a polymer, an electronic structure of the surface thereof, or an electronic structure of a boundary thereof is to be measured, a valence state is observed using an ultraviolet photoelectron spectroscopy and an electron-free state is observed using an inverse photoelectron spectroscopy.
According to a combination of these measuring methods, electronic structures above and below an energy gap which control development of electronic properties can be directly picked up.
A measuring method using a scanning tunneling microscope (STM) or a scanning tunneling spectroscopy (STS) has been known as a method of measuring a local electronic state of a sample in the order of nanoscale.
The STM can control a height of a probe so as to make a tunnel current flowing between the probe and the sample constant, thereby obtaining shape information of the sample and height information thereof. According to the STS, a physical material property can be determined based on variations in voltage applied to a probe and tunnel current value.
There has been known an atomic force microscope (AFM) which is classified as a scanning probe microscope in addition to the STM, for controlling a height of a probe so as to make an intermolecular force acting between the probe and a sample constant, thereby obtaining shape information of the sample and height information thereof.
The intermolecular force used for detection in the AFM is a van der Waals force. This force is a force acting between electric dipoles induced in the probe and the sample and is not significantly changed depending on a material structure in many cases. Therefore, a material surface shape can be suitably measured.
There has been known a measuring method of measuring a current and a voltage using an electroconductive cantilever while measurement is performed by a contact mode atomic force microscope (AFM).
Examples of a force serving as the intermolecular force include not only the van der Waals force and a Coulomb force but also a charge transfer force.
The charge transfer force is a bonding force between molecules which is caused by the partial transport of electrons from an electron donor to an electron acceptor.
The charge transfer force is significantly changed between a material which causes the transport of electrons and a material which does not cause the transport of electrons.
Therefore, it is expected that surface physical properties of various substances can be recognized by accurately measuring the charge transfer force on the sample.
In view of the above, a method of measuring the charge transfer force based on a force curve of the contact mode AFM has been proposed as the method of measuring the electronic state even on the surface of a mixture of a metal and an insulator in Japanese Patent Application Laid-Open No. 2000-028625.
A method of evaluating properties of an insulating film on a semiconductor surface using a probe microscope has been proposed in Japanese Patent Application Laid-Open No. H07-066250.
This method is an evaluating method capable of measuring dielectric breakdown properties of a minute region of a thin film on the semiconductor surface. In the method, an electroconductive probe device is brought into contact with the thin film on the semiconductor surface, and then a temporal variation in the amount of contact electrification amount is measured using the atomic force microscope.
Further, a Kelvin probe force microscope using a non-contact atomic force microscope, for measuring a surface potential of a sample has been known.
In the Kelvin probe force microscope, a contact potential difference caused by a difference between a work function of a probe device and a work function of the sample is detected by the non-contact atomic force microscope.
According to the Kelvin probe force microscope, when a voltage in which a direct current component and an alternating current component (angular frequency ω) are superimposed on each other is applied between the probe device and the sample, the direct current component of the applied voltage is feedback controlled such that an amplitude of an ω component of an electrostatic force acting on the probe device becomes zero, whereby the contact potential difference can be detected.
In other words, while the feedback control is performed, a relative in-plane positional relationship between the probe device and the sample is checked by scanning. Therefore, a two-dimensional distribution on the surface of the sample can be obtained.
A method of measuring a surface shape of the sample and a surface potential thereof based on the principle of the Kelvin probe force microscope has been proposed in Japanese Patent Application Laid-Open No. 2000-329680.
According to the method, when unevenness of the surface of the sample is to be measured, an alternating signal for vibrating a probe and an alternating voltage signal for measuring the surface potential information of the sample are simultaneously applied. When the surface potential information of the sample is to be measured, only the alternating voltage signal for measuring the surface potential information is applied.
Therefore, a frequency component unnecessary at the time of measuring the surface potential information of the sample is suppressed to improve an S/N ratio of the surface potential information of the sample to be measured.
As described above, the various methods for measuring the electronic state density of the surface of high-resistance materials such as a polymer material or an organic material have been proposed up to now.
However, the number of methods which are practically used or widely used is small. Many methods can be applied to only a few materials, or require complicated measurement and take a very long time. According to the method described in J. Appl. Phys. 48 (1977), 4256 or J. Vac. Sci. Tecnol., 3(1985), 732, it is necessary to prepare a plurality of metal materials having different work functions, measurement is complicated, and energy resolution of data does not become constant. According to the above-mentioned evaluation method using the combination of the X-ray photoelectron spectroscopy (XPS) or the ultraviolet photoelectron spectroscopy (UPS) and the inverse photoelectron spectroscopy (IPES), it is necessary to measure the sample in a high-vacuum environment, and the sample is likely to be damaged or charged up. In particular, the charge up causes dulling of a spectral shape or a peak shift, thereby significantly reducing reliability of data.
In order to prevent such a phenomenon from occurring, for example, measures for reducing an intensity of excitation light are required. However, even when the measures are employed, a measurable film thickness of an organic material is up to several tens of nm.
Therefore, not an actual sample but a model sample is made and evaluated in many cases. For example, an extremely thin film is formed on a metal.
The measuring method using the scanning tunneling microscope (STM) or the scanning tunneling spectroscopy (STS) requires feedback of a tunnel current flowing between the probe and the sample, so that only an electroconductive sample can be measured.
Therefore, the sample is limited to a metal, graphite, a semiconductor having a resistance value of several Ω or less, or an extremely thin film formed on a metal.
In addition, because a device structure itself contains a metal and an insulator, it is difficult to measure a local electronic state of the sample to be measured by the existing STM/STS.
The method of obtaining the shape information of the sample and the height information thereof using the atomic force microscope (AFM) is suitable for measuring a material surface shape. However, in order to distinguish materials having different structures from one another or to measure a local electronic state, it is necessary to use a probe having a special function.
In the case of the method of measuring a current and a voltage using the electroconductive cantilever while performing measurement by the contact mode atomic force microscope (AFM), the probe is always in contact with the sample.
Therefore, the electronic state of the sample cannot be measured, so that a transport phenomenon of the entire system which includes a contact characteristic between the probe and the sample to be measured is evaluated.
In the case of measurement using the contact mode AFM, a material with weak attachment to a substrate, such as a carbon nanotube or DNA is pushed away by the probe during measurement, whereby an object type to be measured is limited.
The measuring method described in Japanese Patent Application Laid-Open No. 2000-028625 requires that each focus curve be analyzed for each measurement point to perform evaluation.
Therefore, this measurement takes a very long time and it is difficult to measure an electronic state of a sample fixed at a certain point in the order of nanoscale or to form an image of a result obtained by the measurement. According to the evaluation method described in Japanese Patent Application Laid-Open No. H07-066250, only a dissipation process of injected charges into the semiconductor is measured, and thus, the electronic state of a high-resistance material cannot be measured.
The measuring method using the Kelvin probe force microscope which is described in Japanese Patent Application Laid-Open No. 2000-329680 is intended to improve the S/N ratio of the measured surface potential information when the surface potential information of the sample is detected, and attention is not paid to the accurate measurement of the electronic state density of the surface of the high-resistance material.
In particular, a method of accurately measuring information of a local site and a distribution thereof has been demanded along with extremely in a device. However, a way to realize the method is not taken into account.